Proceedings of ICRC 2001: 2385 c Copernicus Gesellschaft 2001

ICRC2001
ICRC2001

Upper limits on the isotropic gamma ray / cosmic ray ratio from theUpper limits
on the isotropic
gamma ray / cosmic ray ratio from the Grapes
GrapesIII experiment
at Ooty
III experiment at Ooty.

Y. Hayashi1 , S. K. Gupta2 , N. Ito1 , A. Jain2 , S. Kawakami1 , H. Kojima3 , D. K. Mohanty2 , T. Nonaka1 , S. Noto1 , K. C.
1Ravindran21, M. Sasano12, K. Satomi, K. Sivaprasad2 , H. Tanaka1 , T. Toyofuku1 , S. C. Tonwar2 , K. Viswanathan2 ,Y. Hayashi , S.K.1Gupta , N. Ito1, A. Jain2, S. Kawakami1, H. Kojima3, D.K. Mohanty2, T. Nonaka1,andT. Yoshikoshi
1
S. Noto, K.C. Ravindran2, M. Sasano1, K. Satomi1, K. Sivaprasad2, H. Tanaka1, T. Toyofuku1,12
2
1Graduate
SchoolScience, OsakaUniversity
S.C.
Tonwar, K.ofViswanathan
and CityT. Yoshikoshi
Tata Institute of Fundamental Research
32 Tata Institute of Fundamental Research,Nagoya Women s University3
Nagoya Women’s University

2 1Graduate School of Science, Osaka City University,Abstract. We have attempted to determine the isotropic
gamma-ray / cosmic ray ratio using the very large area(560m2) and packed muon detector operating with the EAS
array at Ooty in southern India. The showers with theircores incident within the area of the EAS array but with noassociated penetrating track in the muon detectors,
classified as muon-poor showers, have been considered as
primary gamma-ray initiated shower candidates. We haveobtained upper limits on the flux of isotropic gamma-rays
over the energy region from 20 TeV to 500 TeV. A
conservative upper limit on the ratio, Igamma/ICR of 1*10-3 at
90% C.L., over the energy region, 100 TeV to 500 TeV,
has been placed.

1 Introduction
It is important to estimate the gamma ray intensity apartfrom the total cosmic ray intensity in all energy ranges forvarious studies in astrophysics. In the VHE/UHE region, it
can be estimated by looking at the excess of intensity fromthe direction of a point source which has been confirmed
by observations at other wave lengths, if the astronomical
object exists at relatively short distance. Gamma rays
coming from sources at cosmological distances, such as
distant AGN’s, and gamma rays due to cosmological origin
are expected to arrive isotropically. In several tens of TeV
region, the isotropic gamma ray flux is expected to behigher as a result of cascading from interactions of
extremely high energy cosmic rays with the cosmic
microwave background radiation by the Bottom-Up model(Halzen et al, 1990) or the Top-Down model (Aharonianet.al., 1992). Though, it is experimentally very difficult todistinguish gamma-rays from cosmic rays with groundbased observations, recently upper limits have been placed
Correspondence to: Hayashi(hayashi@sci.osaka-cu.ac.jp)

by several groups which seem to question the validity ofsuch models in the several tens of TeV to several tens of
PeV energy range (Matthews et al 1991, Karle et al 1995,
Aglietta et al 1996, Chantell et al 1997).

2 Detectors
The GRAPES III air shower experiment is located at Ootyin southern India (N 11.4, E 76.7 and 2200m altitude). It
is observing 2 components of air showers, the electromagnetic component and the muon component, to study
cosmic gamma rays and particles in the several tens of TeV
to several tens of PeV energy range. About 250 electron
detectors (scintillation counters) are arranged in a 8m spanhexagonal dense array. The scintillation counter consists ofa 5cm thickness, 1m2 area plastic scintillator viewed by a
5cm diameter photo-multiplier. The counting rate of eachdetectors are monitored continuously.
The muon detectors are arranged in the form of 16
modules with the cluster located towards the northern edgeof the electron detector array. Total muon detection area is560m2. A muon module consists of 232 proportionalcounters (cross-sectional area 10cm x 10cm and length of6m) arranged in 4 layers separated by 15cm thick concrete
layers under 2m thickness concrete absorber giving 35m2
detection area for muons > 1GeV.
The triggering condition is any 10 detectors out of 120
detectors at center of array. The trigger rate is about 13Hz.

3 Data Analysis
We analyzed data observed in last one year, from March
2000 to April 2001. The electron-magnetic component data2386
are used for estimation of primary cosmic ray energy andcore location of air shower. The muon component data areused to select candidate of gamma ray primary.
60

West - East [m]40200$%# $%" 
X ] %$ ! 
[P\YZ $% XPV W $% NM# 
ST U R" O PQN! LM 
-20y a z n k h p c/b{ jsuc}|j ~h p a j _-40

"^ _/`a bcedfc>gh>ij,_>h/klj _/cmonqp `a bcedfc>ghij _/crhs_/ba _/`a bcrj>tfhu7u%hv
monqp `a bcrj>txw=u%u7hv

8 F  32*?@,+E36I8 @3/+8 ?6@A?/6?B,+-G8 0)6A:)6@,+-G-60-100$%
&!
'(*),+-)./+-)02143657+8 .:9 )<;=3/+>8 ? 8 @A?6B,+-)5C+ED? ;8 @F6GIHJ6K-80-60-40-200204060

North - South [m]SC (1m^2)(Trigger)Muon Staion
Acceped Core Location  

3.1 Electro-magnetic component For the electromagnetic component analysis, the following criteria wereused for off-line shower selection for this work:(a) Total number of detected particles > 30.
(b) Detected particles by outer most two rings < 35% of
total detected particles.
(c) Air shower core should be inside a hexagonal area asshown in Fig. 1.
(d) Zenith angle < 25 degs.
(e) Total number of detected particle > 30.
The core location, arrival direction, total number ofdetected particles for electro-magnetic component weredetermined for these showers. The core location isestimated by particle weighted center of higher most 7
scintillation detectors. The arrival direction is estimated byfitting a conical air shower front to the timing data.
Detected number of particles fluctuate quite lot for small
air shower. To avoid larger air shower contamination that
real location is outside of array we used cut (b). This effect
was tested by semi-Monte Carlo simulation through theGENAS program(Ver 2.2, K.Kasahara & S. Torii 1990).
(Fig. 2a, 2b)

 % % %E ° ¯ ±   ­®
   %  E    ,x=,- ,- 4¡£¢6¤o,¥*¦x§6¤¨-©  ª «¬ ²*³´µA¶x³6·¸/¹º ³»Aº ¼I³6½,¹-¼º ¾ µ¿¸6»¾IÀf½¾ Á µ¾I¸6¼Â Ã »¼º ¾6µÂ
²*³´µA¶x³6·¸/¹º ³»Aº ¼º »¼º ¾ µAÄÅµÆ/¸ Á6³»I¸6»6¾IÀ½6¾,Á6µ6¾I¸¼Â Çe½/¹¼ º ¾6µÂÈÉ Ê6Ë ÌxÍÏÎ:ÐÐEÑxÒÓÕÔÐÒxÖÓ-×ÍØ*ÙÅÑxÑÓÑÚ/ÓË

3.2
Muon component: For the muon component
analysis, the detected muon number has been estimatedfrom the observed muon tracks in the 4 layers of thedetector whose direction must match with the direction of
the air shower arrival direction determined from data on
the electro-magnetic component. We use any 3 layers
coincidence out of 4 layers. The number of detected muonsdepends on the air shower primary energy, distance fromthe core to the muon detector, and the muon detection area.
In the Ooty air shower array, typically several muons canbe detected for total number of detected particles 100,
distance from muon detector to core 40m. Though Muon
data recording gate width is 10microsec per event, we used
3.5microsec software gate to reduce chance muons. The

2387

muon counting rate, unrelated to air shower trigger, is~3900/module/sec for any 3-fold coincidences. So,
conservatively, the average number of muons due tochance coincidence is estimated to be ~0.07 per event for
all muon detectors.

we estimated probability of muon less shower by gamma
primary for each distance bin from the center of muon
detectors.

5 Discussion and ConclusionÛsÜâ ã Ü4Ý!"#%$'&)( Ý+*-, . ( , / &)02oÝ+* 
"5 $76 Û98)818

ÛsÜÝ6á 

Ü12xÝ)34ÛsÜÝxÛ þ üEÿüsý

"5 $76+:;$<( , =?>@51ACBÛsÜÝ6àÛsÜÝ ßÛsÜÝ6ÞÛãÛããÛã ã ãÛã ã ããä%åæçèEæxéê ëíìrîïIï}îñðoîòñóoôxõxè ö òø÷ùÅõúû

Û>ã ã ã ã ãUpper limit on the ratio of gamma ray flux to the cosmic
ray flux is given by:I gamma Nmuon − poor (90 %C .L.)11≤⋅⋅
ICRNalle gamma 1 − ⋅nchance
where Nmuon-poor(90%C.L.) is a 90% confidence level upper
limit on the number of muon-poor air showers assuming
Poisson distribution, Nall is the total number of air showers,
egamma is the efficiency to detect gamma ray induced airshowers as muon-poor air showers, nchance is the average
number of muons due to chance coincidence. The result isshown in Fig. 3 and compared with the upper limits givenby other groups.
Present work improves the Igamma / ICR upper limit givenby the HEGRA group for the 50 - 100 TeV region byalmost one order. The Ooty result gives most strict upperlimit for the 20 – 500 TeV region.
Acknowledgement

D"E FG HJILKKMONQPRE STE UWVYX[Z]\%E U)N'VKOE ^4 Simulation and Calculation
It has been known for a long time that gamma ray induced
air showers are muon-poor compared to particle/nucleus
induced showers due to much less number of hadronicinteractions. Monte Carlo simulations with CORSIKA (Ver5.62, Dieter Heck 2000) show that 2 to 3% muons are
expected from gamma ray induced air shower compared toproton induced showers for same primary energy. Since
typical events observed by the Ooty air shower array have0 to several tens of muons, we have classified no-muon
detected air showers as muon-poor events. We havecalculated the upper limit on the ratio of gamma ray flux toall cosmic ray flux using these muon-poor events as thecandidates for gamma ray induced showers. First, weestimated median energy of primary cosmic ray for eachdetected particles bin. We did not find clear difference ofdetected number of particles between proton primary andgamma-ray primary for 10 to a few 100 TeV energy rage.
Same time we got number of muons for each shower andWe are thankful to the Ministry of Education, Japan for partial
financial support for this experiment. We are also happy toacknowledge valuable contributions of G. Paul Francis,V.Jeyakumar, K. Manjunath, K. Ramadass, B.S. Rao, C.
Ravindran, V. Viswanathan and T.Matsuyama during theinstallation, operation and maintenance of the instrumentation.
The help and cooperation of the Radio Astronomy Centre forproviding site facilities for the GRAPES III array are gratefully
acknowledged.

ReferencesM.Aglietta et.al. 1996, Astropart. Phys. 6, 71
F.A. Aharonian et.al. 1992, Phys. Rev. D 46, 4188
M.C. Chantell et.al. 1997, Phys. Rev. Lett. 79, 1805
A.F. Halzen et.al. 1990, Phys. Rev. D 41, 342
A. Karle et.al. 1995, Phys. Lett. B 347, 161
K. Kasahara & S. Torii 1990, ICRR-report 217-90-10
K. Kasahara 1995, 24th ICRC (Rome) 1, 399
J. Matthews et.al. 1991, ApJ 375, 202
M. Sasano et.al. 1999, 26th ICRC